crewman vs water dog

crewman and water dog both are nouns.

Word	Noun	Adjective	Verb	Adverb
crewman	Yes	No	No	No
water dog	Yes	No	No	No

As nouns, water dog is a hyponym of crewman; that is, water dog is a word with a more specific, narrower meaning than crewman:

Other hyponyms of crewman include hand, lascar, gob, Jack-tar, Jack, mariner, old salt, sea dog, seafarer, seaman, tar, water rat, yachtsperson.

crewman (noun)	water dog (noun)
a member of a work crew	a dog accustomed to water and usually trained to retrieve waterfowl
a member of a flight crew	a person who enjoys being in or on the water
any member of a ship's crew